The present invention relates to a device for stirring a liquid in a reactor and for injecting a gas into this liquid, employing a self-priming impeller.
Document EP-A1-0 995 485 describes a device for stirring a liquid in a reactor and for injecting a gas into this liquid. This device comprises a drive motor for driving a vertical shaft, which is arranged above the reactor. The shaft of the motor carries and drives at its lower end a propeller submerged in the liquid; it likewise carries and drives a self-priming impeller placed between the surface of the liquid and the propeller. The self-priming impeller is connected to a source of gas, generally an oxygenated gas, in such a way that, when it is driven by the shaft of the motor, it simultaneously sucks in gas and liquid in which it is submerged, thereby forming a gas/liquid dispersion. The gas/liquid dispersion generated by the self-priming impeller is directed toward the propeller with the aid of a baffle-forming annular casing which envelops the self-priming impeller.
It has been found that, under certain use conditions with this type of prior art device, the capacity to suck gas into the impeller was limited due to the volume defined by the impeller and the annular casing being clogged with gas. Thus, evacuating the gas/liquid mixture from the annular casing can only be done with difficulty: on the one hand, there is no dispersal of the gas into the reactor and, on the other hand, the gas present under the annular casing attempts to escape through the means for admitting the liquid into the impeller, which results in no gas being transferred into the liquid and in the gas being wasted, the gas rising to the surface without being used.